Coaxial cables, such as for microwave transmission, have existed in the prior art for a considerable period of time. As technology has developed, a need for flexible coaxial cables whose electrical characteristics do not vary during flexure of the cable, such as in aerospace utilizations, has developed. In such utilizations, often the electrical characteristics of the cable are critical and any variation therein will yield unsatisfactory transmissions via such cables. In order to increase the flexibility of prior art coaxial cables, corrugated outer conductors, such as disclosed in U.S. Pat. Nos. 3,582,536; 3,173,990 and 2,890,263 have been utilized. In addition, other prior art attempts of providing such flexibility have employed a corrugated outer sheath for the cable rather than a corrugated outer conductor, such as disclosed in U.S. Pat. No. 3,002,047. Furthermore, this concept of a corrugated outer sheath has been utilized for standard electrical cables, as opposed to coaxial cables, where such cables are exposed to considerable flexure, such as disclosed in U.S Pat. Nos. 2,348,641 and 2,995,616.
In order to ensure electrical stability for a coaxial cable, the relative location between the various portions of the outer conductor, the dielectric and the inner conductor must remain constant during flexure of the cable or the electrical characteristics may vary. Prior art attempts to ensure this stability have involved the locking of a corrugated outer conductor to the dielectric surrounding the inner conductor, such as disclosed in U.S. Pat. No 3,173,990 wherein such inner conductor is a foam polyethylene. However, such prior art flexible coaxial cables do not have sufficient flexibility nor do they have sufficient temperature stability, which also affects the electrical characteristics. These prior art coaxial cables utilize either a tube which is crimped to provide a corrugated tube or form the outer conductor by means of helically winding a piece of conductive material, welding the adjacent pieces together to then form a tube and, thereafter, crimping alternate longitudinal portions so as to provide a corrugated tube. In both instances, the maximum pitch for the convolutions of the outer conductor is severely limited. In the first instance, this limitation is primarily due to rupture of the conductive tube if the crimps are too closely spaced together whereas, in the second instance, the limitations are primarily due to the inability to sufficiently control the thickness of the resultant tube which is formed as a thin enough material cannot be utilized to produce a high pitch. Since the higher the pitch of the convoluted outer conductor, the greater the flexiblity of the coaxial cable, these prior art flexible coaxial cables have not been satisfactory where large degrees of flexure are required together with electrical and temperature stability over a wide range of flexure.
Furthermore, these prior art flexible coaxial cable have primarily been of the foam polythylene or solid dielectric type whereas flexible coaxial cables utilizing spline dielectrics have not exhibited satisfactory electrical and temperature stability characteristics.
These disadvantages of the prior art have been overcome to an extent by the prior invention of U.S. Pat. No 3,797,104 employing a solid dielectric. However, the ability to provide flexible coaxial cables for certain applications in which a particular velocity of propagation or lower attenuation was required was somewhat limited as was the ability to readily change the velocity of propagation of the flexible coaxial cable to the desired value during manufacture. Moreover, although there have been prior art attempts to use helically wound dielectrics for coaxial cable, such as disclosed in U.S. Pat. No. 4,346,253; French Patent No. 752,006 and British Patent No. 616,303, they have not been satisfactorily employed for flexible coaxial cables, particularly since any change in pitch of the helically wound dielectric during flexing of the cable would undesirably change the properties of the cable. These disadvantages of the prior art have been overcome to some extent by the prior invention of U.S. Pat. No. 4,758,685 employing a heat shrinkable dielectric tubing surrounding a helically wound dielectric beading. However, the process of manufacturing such a flexible coaxial cable is difficult and necessarily can lend itself to instabilities, such as if the shrinking were non-uniform thereby resulting in a non-uniform dielectric core which could cause problems in inserting the core into the outer conductor, and resultant electrical instability due to the locking of a non-uniform core. These disadvantages of the prior art are overcome by the present invention which provides an easily controllable manufacturing process, which yields a highly stable flexible coaxial cable in which the characteristics of the cable can readily be controlled during the manufacturing process.